1. Field of the Invention
The invention relates generally to an apparatus for retaining a workpiece within a semiconductor wafer processing system and, more specifically, to an improved composition of a electrostatic chuck that maximizes electrostatic clamping ability without loss of material strength or modulus of elasticity.
2. Description of the Background Art
Electrostatic chucks are used for retaining a workpiece in various applications including retaining a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. The pedestal may form an electrode and a heat sink or heater as used in etching, physical vapor deposition (PVD) or chemical vapor deposition (CVD) applications. For a detailed understanding of the reaction chamber and its operation in processing the wafer, the reader should refer to the drawings and the detailed description contained in U.S. Pat. No. 5,228,501, issued to Tepman et al. on Jul. 20, 1993, and incorporated herein by reference. That patent teaches a PVD wafer-processing chamber manufactured by Applied Materials, Inc. of Santa Clara, Calif. Additionally, the operation of a conventional electrostatic chuck is disclosed in U.S. Pat. No. 5,350,479, issued to Collins et al. on Sep. 27, 1994 assigned to the assignee hereof, and its disclosure is incorporated herein by reference.
The mechanism of attraction in the electrostatic chuck used in these types of wafer processing systems is generally a Coulombic force. That is, the increase of charges in an insulated electrode induces opposite charges to gather on the backside of the wafer. The resultant force is generally weak per unit area i.e., 15 g/cm2 at 1500V DC because of the composition of the chuck. For example, a commonly used type of dielectric material for fabricating electrostatic chucks is polyimide. Specifically, electrodes are usually sandwiched between two sheets of polyimide to form an electrostatic chuck. Among the beneficial characteristics of polyimide are its high strength and high modulus of elasticity. This material also has high volume resistivity (on the order of 1015 ohm-cm) and surface resistivity (on the order of 1014 ohm/cm2). Since the electrode(s) are insulated and a high resistivity dielectric is used, the charges creating the chucking force are not mobile i.e., the dielectric layer separates the electrode and wafer. As such, the wafer must come into contact with a large area of the chuck so that an adequate charge accumulation is established for wafer retention.
One example of an improved electrostatic chuck is one that employs the Johnsen-Rahbek (J-R) effect and can be found in U.S. Pat. No. 5,463,526, issued Oct. 31, 1995 to Mundt. In such a chuck, the dielectric material has an intermediate resistivity instead of a high resistivity. As such, there are mobile charges present in the dielectric material. These mobile charges create a small but effective current flow between the backside of the wafer and Electrostatic chucks using the J-R effect are usually fabricated from a ceramic having an intermediate or xe2x80x9cleakyxe2x80x9d dielectric characteristic. Materials such as aluminum nitride, and silicon oxides the top surface of the electrostatic chuck.
Oxides and nitrides are popular and well known for use in electrostatic chucks. However, these types of materials, and specifically aluminum nitride, become increasingly conductive after prolonged processing exposure at high temperatures such as 500xc2x0 to 600xc2x0 Celsius. As such, more mobile charges are able to pass through the chuck material and into the wafer, thereby lessening the accumulation of the mobile charges within the top surface of the electrostatic chuck and the backside of the wafer. As a result, the electrostatic force across the chuck surface is weakened, thereby reducing the chucking capabilities of the electrostatic chuck. Furthermore, in extreme cases, the wafer may become damaged due to the excessive current flow through the chuck and wafer.
Therefore, there is a need in the art for an improved apparatus for retaining a wafer, but have a reduced resistivity level so as to take advantage of the J-R effect for clamping the wafer. Additionally, such an apparatus should be simple and cost-effective in design and construction to properly retain the wafer or workpiece. Moreover, such an apparatus must be able to withstand repeated processing cycles without deteriorating, that is, exhibiting excessive conducting current through the surface of the electrostatic chuck at the expense of weakening chucking forces.
The disadvantages heretofore associated with the prior art are overcome by an apparatus for retaining a workpiece in a semiconductor processing chamber. The apparatus includes a controlled resistivity boron nitride plate and at least one chucking electrode embedded in the controlled resistivity boron nitride plate. Optionally, a heater plate, fabricated from boron nitride and having at least one heater element embedded therein, is disposed beneath the controlled resistivity boron nitride plate to provide temperature regulation of the apparatus during semiconductor processing.
A first method for fabricating the apparatus includes providing the controlled resistivity boron nitride plate. A conductive layer is disposed on a portion of a lower surface of the controlled resistivity boron nitride plate to form the at least one chucking electrode. A layer of boron nitride powder is disposed on the conductive layer and the lower surface of the controlled resistivity boron nitride plate. Thereafter, the controlled resistivity boron nitride plate, conductive layer, and boron nitride powder are hot pressed together to form the apparatus.
A second method for fabricating the apparatus includes providing a controlled resistivity boron nitride plate. At least one conductive layer is deposited on a portion a lower surface of the controlled resistivity boron nitride plate to form the at least one chucking electrode. A layer of pyrolytic boron nitride is deposited on the conductive layer and the lower surface of the controlled resistivity boron nitride plate to form the apparatus.